[chore] reuse struct when parsing to avoid recursive parsing (#31727)

**Description:** 
This is a companion PR to handle recursive state of unmarshalers with
open-telemetry/opentelemetry-collector#9750.
Changing this behavior will allow the confmap.Conf object to recognize
that it has already run the `Unmarshal` method on the struct, and run
the mapstructure decoding of fields.

exporter/datadogexporter/config_warnings_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/confmap"
 )
 
@@ -79,7 +80,7 @@ func TestSendAggregations(t *testing.T) {
 		t.Run(testInstance.name, func(t *testing.T) {
 			f := NewFactory()
 			cfg := f.CreateDefaultConfig().(*Config)
-			err := cfg.Unmarshal(testInstance.cfgMap)
+			err := component.UnmarshalConfig(testInstance.cfgMap, cfg)
 			if err != nil || testInstance.err != "" {
 				assert.EqualError(t, err, testInstance.err)
 			} else {
@@ -156,7 +157,7 @@ func TestPeerTags(t *testing.T) {
 		t.Run(testInstance.name, func(t *testing.T) {
 			f := NewFactory()
 			cfg := f.CreateDefaultConfig().(*Config)
-			err := cfg.Unmarshal(testInstance.cfgMap)
+			err := component.UnmarshalConfig(testInstance.cfgMap, cfg)
 			if err != nil || testInstance.err != "" {
 				assert.EqualError(t, err, testInstance.err)
 			} else {

exporter/datasetexporter/config_test.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/config/configretry"
 	"go.opentelemetry.io/collector/confmap"
 	"go.opentelemetry.io/collector/exporter/exporterhelper"
@@ -22,7 +23,7 @@ func TestConfigUnmarshalUnknownAttributes(t *testing.T) {
 		"api_key":           "secret",
 		"unknown_attribute": "some value",
 	})
-	err := config.Unmarshal(configMap)
+	err := component.UnmarshalConfig(configMap, config)
 
 	unmarshalErr := fmt.Errorf("1 error(s) decoding:\n\n* '' has invalid keys: unknown_attribute")
 	expectedError := fmt.Errorf("cannot unmarshal config: %w", unmarshalErr)

exporter/signalfxexporter/config_test.go

@@ -552,7 +552,7 @@ func TestUnmarshalExcludeMetrics(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			require.NoError(t, tt.cfg.Unmarshal(confmap.NewFromStringMap(map[string]any{})))
+			require.NoError(t, component.UnmarshalConfig(confmap.NewFromStringMap(map[string]any{}), tt.cfg))
 			assert.Len(t, tt.cfg.ExcludeMetrics, tt.excludeMetricsLen)
 		})
 	}